The pinning of magnetic flux lines in anisotropic high-temperature superconductors is an area of active research. In Bi-based materials, weak flux-line pinning above intermediate temperatures (T&gt;20K) severely constrains potential magnet applications and restricts liquid-nitrogen temperature applications to a very low magnetic field (&lt;IT). In order to overcome these limitations, it is desirable to discover flux-pinning methodologies for introducing effective pinning centers in materials potentially useful for commercial applications, such as, but not limited to, thin films, thick films, wires and/or tapes.
Increased transport critical currents in high-temperature superconductors is a primary goal of high-temperature superconductor physics and engineering. The ability to increase the current greatly raises the potential use of high Tc superconductors in a wide range of applications.
Progress in this regard has been reported concerning very high magnetic moments in superconducting single crystals and polycrystals such as YBa.sub.2 Cu.sub.3 O.sub.7 (Y123) with Y211 precipitates of approximately 0.1 .mu.m diameter. Amorphous tracks of approximately 10 .mu.m diameter and 1 to 10 .mu.m length created by the use of heavy ion bombardment has been shown to substantially increase flux line pinning in Y123 and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x (Bi2212) superconductors. Proton irradiation with a subsequent decay of Bi nuclei created amorphous tracks in BiSrCaCuO (BSCCO family) similar to the heavy ion tracks, but of shorter length. Neutron irradiation of HgBa.sub.2 CaCu.sub.2 O.sub.6+x has been shown to increase the magnetization hysteresis as well as to considerably lift the irreversibility line. Li doping has been shown to enhance collective pinning.
Each of the above techniques, except for Li doping, has practical difficulties when contemplating commercial applications. For example, it would be extremely difficult to transfer the method of making high magnetic moment superconductor for use with arbitrary shapes or wires. The use of heavy ion bombardment in large scale applications is virtually unimaginable. The unavailability of accelerator energies for proton irradiation for large scale application is a limiting factor as well as the problem of radioactivity in the silver sheath of the tapes. Similarly, neutron irradiation has practical problems for commercial application.
Therefore, most promising work appears to be in the field of embedding defects or dopants into a superconductor matrix, preferably by premixing the components, followed by a reaction process or alternatively, precipitating nanoparticles during processing as was achieved in Y123 crystals with Y211 precipitates.
The preferred method, in most cases, is to premix nanoparticles or atomic constituents into a precursor powder followed by standard processing procedures or variations thereon, for example, calcination, melting and oxygenation.
The achievement of better pinning in high-temperature superconductors in the described manner will result in several advantages, including industrial scale-up and shape problems being obviated, the anisotropy of magnetic properties being substantially diminished and critical currents being achieved for a wider range of applications, provided that the additives do not result in a deterioration of the contact between grains.
The type of particles needed for achieving these results would be of a shape and a dimension similar to the columnar tracks obtained from heavy ion irradiation. Moreover, the particles have to be stable in the very aggressive chemical environment at the elevated temperature required to optimize superconductivity in the matrix material. The content and composition of the particles, once embedded in the matrix, might not be critical (barring strong ferromagnetism).
The primary object of the method is to displace superconducting matrix material from a volume suitable for achieving strong pinning over some length of the magnetic vortices penetrating the material. The comparison with columnar defects created by ion tracks is that ion tracks are an ill-defined amorphous substance, in all probability not superconductive at all.
In an article by P. Calvert entitled "Strength in disunity," in Nature, vol. 356, Jun. 4, 1992, page 365, the use of polymer composites with tubular fullerenes having diameters of a few nanometers and a length of up to several micrometers, i.e. carbon nanotubes, for increased structure strength is described.
The present invention provides enhanced flux pinning in superconductors by embedding carbon nanotubes in the superconducting matrix.
In order to accomplish the desired results, particularly with regard to size and shapes, carbon nanotubes are embedded in the superconductor in an effort to match the required electrical requirements with the need to survive the hostile environment to which the nanotubes would be exposed or that the volume displaced by the nanotubes remains a non-superconducting region (as a void or blended defect) even if the nanotubes themselves are fully or partially consumed. The major problems to be overcome are wettability of the nanotubes, a lack of stability of the nanotubes and/or displaced volumes (i.e. collapsing of any voids or disappearance of extended defects created by the fully or partially consumed nanotubes) at elevated temperatures, and the detrimental influence the reaction between carbon and oxygen could have in view of literature evidencing such an effect.
The use of carbon nanotubes meets the requirements of shape, size, availability and scale-up potential. In addition, in highly anisotropic materials of the Bi-family the flux lines are pinned by columnar defects over a wide range for the relative orientation of the defects and the magnetic field. Therefore, by embedding particles having the shape of nanotubes and oriented at random, the magnetic anisotropies would be expected to be greatly reduced. The possibility of large scale production of nanotube material is described in an article by T. W. Ebbesen et al entitled "Large-scale Synthesis of Carbon Nanotubes" in Nature, vol. 358, Jul. 16, 1992, pages 220 to 222.
Thus, the embedding of carbon nanotubes in superconducting material, and particularly in the BSCCO family of superconducting material, and specifically in Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.s+x superconducting material, results in superconducting material exhibiting enhanced flux pinning. Other BSCCO family members include (Bi, Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.10+x.